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    <title>ShellCheck: SC3016 – In POSIX sh, unary `-v` (in place of `[ -n "${var+x}" ]`) is undefined.</title>
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    <h1>SC3016 – ShellCheck Wiki</h1>
    <a href="https://github.com/koalaman/shellcheck/wiki/SC3016">See this page on GitHub</a>
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    <h2 id="in-posix-sh-unary--v-in-place-of---n-varx--is-undefined">In
POSIX sh, unary <code>-v</code> (in place of
<code>[ -n "${var+x}" ]</code>) is undefined.</h2>
<h3 id="problematic-code">Problematic code:</h3>
<div class="sourceCode" id="cb1"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb1-1"><a href="SC3016.html#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="co">#!/bin/sh</span></span>
<span id="cb1-2"><a href="SC3016.html#cb1-2" aria-hidden="true" tabindex="-1"></a><span class="cf">if</span> <span class="bu">[</span> <span class="ot">-v</span> STY <span class="bu">]</span></span>
<span id="cb1-3"><a href="SC3016.html#cb1-3" aria-hidden="true" tabindex="-1"></a><span class="cf">then</span></span>
<span id="cb1-4"><a href="SC3016.html#cb1-4" aria-hidden="true" tabindex="-1"></a>  <span class="bu">echo</span> <span class="st">&quot;STY is set, you are using screen&quot;</span></span>
<span id="cb1-5"><a href="SC3016.html#cb1-5" aria-hidden="true" tabindex="-1"></a><span class="cf">fi</span></span></code></pre></div>
<h3 id="correct-code">Correct code:</h3>
<p>Either switch to bash or ksh, or use:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb2-1"><a href="SC3016.html#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="co">#!/bin/sh</span></span>
<span id="cb2-2"><a href="SC3016.html#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="cf">if</span> <span class="bu">[</span> <span class="ot">-n</span> <span class="st">&quot;</span><span class="va">${STY</span><span class="op">+</span>x<span class="va">}</span><span class="st">&quot;</span> <span class="bu">]</span></span>
<span id="cb2-3"><a href="SC3016.html#cb2-3" aria-hidden="true" tabindex="-1"></a><span class="cf">then</span></span>
<span id="cb2-4"><a href="SC3016.html#cb2-4" aria-hidden="true" tabindex="-1"></a>  <span class="bu">echo</span> <span class="st">&quot;STY is set, you are using screen&quot;</span></span>
<span id="cb2-5"><a href="SC3016.html#cb2-5" aria-hidden="true" tabindex="-1"></a><span class="cf">fi</span></span></code></pre></div>
<h3 id="rationale">Rationale:</h3>
<p>Your script uses a shell feature not supported by the shebang. Either
rewrite the script to be portable, or change the shebang to explicitly
require a shell like Bash.</p>
<p>In this case, <code>[ -v variable ]</code>, which checks if a
variable is set, is <a
href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html">not
specified by POSIX</a>. It can be replaced with the portable
<code>[ -n "${variable+x}" ]</code>, which uses the "alternative value
if set" <a
href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02">parameter
expansion</a> syntax to accomplish the same thing.</p>
<h3 id="exceptions">Exceptions:</h3>
<p>None</p>
<h3 id="related-resources">Related resources:</h3>
<ul>
<li>Help by adding links to BashFAQ, StackOverflow, man pages, POSIX,
etc!</li>
</ul>
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